You do not need to warm your fresh juice on a stove as the poster a few above me suggested. That is ridiculous.
Basically, most juice vendors mix a batch explicitly for your order (the PG/VG ratio, etc..). It takes a while for the flavors to settle down, mix, and get right. It is everything mixing together, ala making spaghetti sauce or chili, in which you have to give it a few days for the flavors to get to know each other.
There are many different ways people steep their juices, such as shaking them up everyday or leaving the juice with the top off for 24 hours. It comes down to this: juice just needs to age. The magic number is 7 days; however, depending on the type of juice you get, it could take up to 2-4 weeks. There are many variables to this. Larger bottles take longer to steep. Blends with a lot of VG take longer to steep and tobacco flavors usually take longer to steep. There is no 'right' way to steep. Personally, I just taste test my juice and if it is good, I go with it. If it is perfume-y or tastes chemical-like, I leave it alone for a while.